一国外科研论文--蚊子咬人的力学行为 Mechanics of a mosquito bite.pdf

abstract swaminathan, vinay shankar. mechanics of a mosquito bite. (under the direction of dr. m k ramasubramanian.) the objective of this thesis is to provide a fundamental understading of the process of a mosquito bite. mosquito fascicle, which is the part of the mosquito used for penetrating the skin and drawing blood, is 2mm long and 30 ?m in diameter. a needle of this size to be able to penetrate a tough membrane like the human skin needs some kind of stabilization phenomena with which buckling failure can be prevented. by studying high speed video of the process and scanning electron microscope images of the fascicle, a stability mechanism is observed which increases the critical buckling load of the fascicle and prevents failure. this stability mechanism is explained by using the theory of non- conservative force and becks column theory. to further support the argument of non- conservative forces, finite element models of the skin penetration process is built. since there is no published data on the properties of the fascicle, mechanical tests are conducted to get stress strain data of the fascicle to build the material model used in finite element analysis. the finite element simulations support this concept where the fascicle can only penetrate the skin with the application of non conservative forces. mechanics of a mosquito bite by vinay s swaminathan a thesis submitted to the graduate faculty of north carolina state university in partial fulfillment of the requirements for the degree of master of science in mechanical engineering raleigh, nc 2006 approved by: dr k j peters dr c s apperson dr m k ramasubramanian (chair of advisory committee) biography vinay s swaminathan was born on the 16th day of june, 1981 in an obscure little town called gelsenkirchen in germany. after returning to india, and completing his bachelors in engineering from university of madras, the author joined north carolina state university in the fall of 2003. iii acknowledgments first and foremost i would like to thank my faith in god for giving me the hope in times of despair not only during the course of this project but throughout my life. my mother has played the most important role in my life. without her my life would not be at this present juncture. i thank her for her strength, support and love. i would also like to express my profound gratitude to dr. ramasubramanian for giving me the opportunity to work on this project and providing me with the guidance to complete it and helping me find the right direction when it seemed that there were none. i hope i am able to justify his belief in this project and me. i also would like to thank dr. apperson for his support during the course of the project and knowledge in the field of mosquitoes, which was a complete unknown before this project commenced and dr. peters for evaluating the project as a committee member. my heartfelt gratitude extends to my sister and brother in law who never made me feel homesick in a new country and have been my pillars of support over the past 2 years. none of this would have been possible without the guidance of the above-mentioned people and without the support of my research group. no life is complete without friends and i like to thank all the people i have had an opportunity to know, for bearing through my quirks during some tough times. and finally i would like to thank a special person called ramya for her undying support, confidence, patience, and love and for making me look forward to the future with anticipation. iv table of contents list of figures vi list of tables.vii chapter 1. 1 introduction:. 1 1.1 background 1 1.2 theory of failure in microneedles 2 1.3 microneedle fabrication. 3 1.4 blood feeding in mosquitoes. 5 1.5 structure of the fascicle. 6 chapter 2. 8 physical characterization of a mosquito biting parts 8 2.1 scanning electron microscope imaging. 8 2.2 high speed video imaging of the mosquito feeding process. 14 2.2.1 observation and initial conclusions. 14 2.3. application of non-conservative force 18 chapter 3. 21 stability mechanisms. 21 3.1. theory of elastic foundation 21 3.2 theory of non-conservative force 23 3.3. methods of application:. 28 chapter 4 . 30 mechanical testing of fascicle. 30 4.1 introduction . 30 4.2 experimental protocol . 30 4.2.1 sample preparation. 30 4.3 test set up. 33 4.3.1 challenges in testing. 34 4.4 results and conclusions 36 4.4.1 theory of hyperelasticity 21 38 chapter 5. 41 finite element analysis . 41 5.1. introduction 41 5.2. skin model . 42 5.3. contact model 44 5.4. element formulation 45 5.5. results . 46 chapter 6. 53 conclusion: 53 list of references . 54 v list of figures figure 1:5 concept of a blood monitoring device 2 figure 2: 9 machined microneedle 4 figure 3:sem image of the mosquito showing various parts. 6 figure 4:female fascicle showing reinforcement at the tip 9 figure 5:. male fascicle tip. 10 figure 6:sem image of fascicle showing surface detail 11 figure 7: cross sectional sketch of the fascicle. 13 figure 8:sketch of the fascicle showing the dimensions in mm. 13 figure 9:this still image shows the probing process during feeding. 15 figure 10: this image shows that the large impulsive force applied during the probing process is partly due to forward motion of the whole body applied by lifting and pushing of the legs 15 figure 13: a becks column . 24 figure 14: plot of equation 3 27 figure 15: solution to becks column 28 figure 16: sample loading on a paper strip 32 figure 17: enduratec load frame 33 figure 18: daq set up. 34 figure 19: stress strain curve for fascicle 37 figure 20: hyperelastic curve fit for fascicle data 38 figure 21: layer of the skin 13 44 figure 22: buckling of the needle with application of axial displacement. 47 figure 23: load displacement with 0.1n follower force. 48 figure 24: contact stress with 0.1n follower force. 49 figure 25: load displacement with 0.5n follower force. 50 figure 26: contact stress with 0.5n follower force. 50 figure 27: load displacement for 1n follower force 51 figure 28: contact stress with 1n follower force. 51 vi list of tables table 1: skin mechanical properties 26. 43 vii chapter 1 introduction: 1.1 background development of micro needles for several biomedical applications has quickly become one of the fastest growing research fields today. with more and more people getting affected with diabetes, micro needles present a very high potential in blood drawing for monitoring and drug delivery 123. patients including infants need continuous monitoring of their blood sugar levels, which so far involves a drop of blood collected from the finger tip and fed into the meter. this process is extremely painful because of the lancet used to prick the finger for the blood sample. in fact repeated use of this procedure leads to numbness in that part and patient trauma. the main idea behind the microneedle approach is that due to the small size of the needles, tissue damage will be limited and pain sensation can be reduced 4 or even completely avoided. over the past decade, the use of micro needles has been suggested for blood drawing to reduce this sensation of pain. giorgio e. gattiker, karan v. l s. kaler et al. 5 designed a mems device for blood glucose monitoring using the similar drawing technique as the blood drawing in mosquitoes. however no investigation was done on the stability of their microneedle which had dimensions of, significantly bigger than a mosquito fascicle. 1 figure 1:5 concept of a blood monitoring device one of the major obstacles in this application is with the possibility of needle breakage which can lead to severe complication in patients. 1.2 theory of failure in microneedles when a microneedle penetrates a tough membrane, there is a short period of time when the needle is under high compressive stress. during this period, if the second moment of area of the microneedle is not high enough relative to its length, the microneedle will buckle. for buckling analysis of microneedles, it is necessary to determine whether the buckling will be short column buckling or long column buckling. for the euler equation for long elastic columns of length l to be applicable the slenderness ration l/k must be greater than the critical slenderness ratio 6. cr f ea kl 4 )/( 2 ? ? 2 where e is the youngs modulus, a is the area and is the critical force that will cause short column buckling. the radius of gyration k is defined as cr f a i k ? where i is the second moment of area. thus the critical slenderness ratio is defined as y cr e k l ? ? 2 )( 2 ? the slenderness ratio for a typical microneedle is 114. thus microneedles can be modeled as long columns and the euler equation for elastic buckling can be applied. 2 2 l eic f ? ? zahn et al. 7 were also able to show that shear could be another possible mode of failure, and the maximum shear force v was approximated as 2 a v y ? ? however in their study they found this value of shear force required for failure a lot higher than the buckling force, which indicates that failure buckling modes were more likely to occur. 1.3 microneedle fabrication different materials have been used in microneedle fabrication. polysilicone needles made of ceramics have shown good potential. however ceramic needles fail due to crack initiation and propagation. a number of techniques have been used for producing smaller and smaller needles including using silicon fabrication 7 and microneedle 8 arrays, the latter applied to transdermal drug delivery. gardeniers, regina luttge et al. 9 used micro injection molding technique to fabricate plastic needles. the advantages of plastic microneedles 3 are that they are easily disposable, can be melted at lower temperatures and are flexible enough to be used for intravenous catheters. however they suffer from lack of strength and easy failure and the use of plastic for needles is still under investigation. figure 2: 9 machined microneedle as mentioned earlier, giorgio gattiker, karan v. l s. kaler et al.5 have been able to manufacture microneedles with base width of 100 ?m and hole diameter of 50 ?m using silicon. silicon fabrication basically involves 8 taking a silicon wafer and growing a layer of silicon dioxide of thickness 0.7 ?m and patterning it using a deep reactive ion etching (drie) technique. this process forms the channel with the desired width and height in the silicon wafer. the residual silicon dioxide is removed, followed by necessary cleaning of the wafers before the actual bonding takes place. the resulting wafer is fusion bonded with another silicon wafer. once the two wafers are bonded, the top wafer is subsequently thinned down, to achieve the desired top wafer thickness. the bonded wafers are then patterned according to the desired shape and length and etched to obtain a vertical structure with high aspect ratio. finally, patterned backside etching of the microneedle is carried out, to achieve the desired thickness. 4 nature, on the other hand has been able to produce a microneedle system with 100% reliability and with a totally painless blood drawing capability. a female mosquito is able to feed on human blood without producing a sensation of pain and without any failure of its needle or fascicle. the only sensation and that of itching is produced due to the introduction of mosquito saliva which prevents clotting of the blood. the immediate aim of this project is to understand the mechanics involved in the needle insertion process, to completely characterize all aspects of this unique process and explore ways to mimic this system for a novel blood drawing device which can be used for glucose monitoring or for drug delivery in a painless manner. a female mosquito is able to draw as much as 2.5 microliters of blood which makes the system more attractive as this amount is adequate for blood glucose monitoring. taking the concept one step further, the system could be reversed to inject insulin or other therapeutic agents back into the body when necessary in painless manner 1.4 blood feeding in mosquitoes. the fascicle of the aedes aegypti, a common type of mosquito as observed from the scanning electron microscope pictures are 2mm long and around 30 ?m in width. therefore there are much too large to enter the capillaries (10-20 ?m) located just below the epidermis in humans. they can however enter the larger arterioles and venules which are of diameter 40-60 ?m in the upper and mid dermis and 100-400 ?m in the deeper tissues. 5 figure 3:sem image of the mosquito showing various parts 1.5 structure of the fascicle the fascicle comprises of 6 different stylets 10 which are the labrum, the paired mandibles, the hypopharynx, and the paired maxillae. in mosquitoes the labrum is the largest, stiffest and most dorsal stylet in the fascicle. the tip of the labrum curves downwards slightly and is sharpened-off ventrally like a quill pen. the mandibles are extremely thin, delicate stylets. at the base of the proboscis the mandibles occupy a lateral position in the fascicle. in ae. aegypti the mandibles form the ventral closure for the salivary canal. the hypopharynx is a delicate flat unpaired stylet with a thickening along its midline which contains the salivary canal. the maxillae are the principle piercing organs, are highly modified, each consisting of an internal rod, a stylet, a pal panda complex musculature. near the base of the proboscis the paired maxillary stylets are lateral in location but over most of the length of the proboscis they lie side by side. the sclerotized tips of the maxillary stylets are sharply tapered and in most species their curved outer edges bear backward pointing teeth, and in some species the inner edges bear a few forward pointing teeth. the outer edges of the mandibular and the maxillary stylets interlock with the lateral grooves of the labrum. because of the strength and 6 curved form of the maxillary stylets it has been suggested that it is the interlocking of the maxillary stylets and the labrum that holds the several stylets in the fascicle together 11 12 and 13. it is believed that it is the movements of the maxillae which are responsible for penetration of the fascicle, with the maxillary teeth acting as the grappling hooks. the movement of the right and left protractor and retractors (which are anchored to the skin by its teeth) pulls the cranium towards the skin surface and pushes the fascicle deeper into the skin. once the penetration is done, the tip of the fascicle has been observed to bend through almost a right angle. there is a very limited sideways movement. mellink et al. 14 were alone in considering that the fascicle is uniformly stiff and inflexible, that it only bends passively and in response to host tissue resistance. however movement of the tip of the fascicle is ascribed to the contraction of various muscles in the labrum. this project aims at investigating the mechanism of the skin penetration process, and understand the reasons for stability in mosquito fascicle which at very small dimension is able to penetrate the skin and withdraw blood. to be able to manufacture a needle of that dimension and integrate this needle with a blood glucose monitoring system is the long term goal of the project. 7 chapter 2 physical characterization of a mosquito biting parts 2.1 scanning electron microscope imaging the first step in understanding a mosquito feeding process was to first understand the geometry of the fascicle. the questions posed were how big a fascicle is, does it have any special geometry at its tip, is there a physical reason that only the female mosquitoes feed. for this reason scanning electron microscopy was used. male and female proboscises were extracted from mosquitoes and then sem images were obtained. initially it was hoped that the sem images would also reveal some surface details of the fascicle. as predicted, the female fascicle and male fascicle had the same dimensions. however there was a stark contrast in the fascicle tip geometry. 8 figure 4:female fascicle showing reinforcement at the tip figure 5. close up sem image of the female fascicle tip 9 the above sem image (fig.4) clearly shows the sharp hypodermic needle like tip geometry. the close up image of the fascicle tip also shows a reinforcement structure at the tip. in contrast to this, a male fascicle (fig.5) has a blunt, scoop like geometry which it has adapted for flower nectar feeding. figure 5:. male fascicle tip as mentioned earlier, it was also hoped that some surface detail could be obtained from the sem images. however, due to the necessity of surface preparation to get a high resolution clear image, the sem details didnt reveal too much of the surface characteristic as is clear from highly magnified image of the female fascicle (fig.3) 10 below. however not only did the sem images help in establishing the difference in the female and male fascicles, but it helped in getting information on the fascicle geometry. on an average the mosquito fascicle was found to be 2mm in length. the fascicle is a hollow tube with an average wall thickness of 4-5 ?m. at its widest, the fascicle has an outer diameter of 30 ?m which tapers to form the sharp tip shown in figure above. figure 6:sem image of fascicle showing surface detail one of the main objectives of the project is to understand the mechanics involved in the mosquito feeding process and thus the sem images brought forward the first challenge, to understand how the fascicle 2mm long and with an outer diameter of 30 ?m at its 11 widest, is able to penetrate human skin and draw blood without any form of structural failure. thus evolves the need to understand the structural composite which forms the mosquito fascicle. the sem images also show the smooth exterior surface of the fascicle with uniform ridges along the whole length. a